| Ia | Evidence from systematic reviews or meta-analysis of randomised controlled trials |
| Ib | Evidence from at least one randomised controlled trial |
| IIa | Evidence from at least one controlled study without randomisation |
| IIb | Evidence from at least one other type of quasi-experimental study |
| III | Evidence from non-experimental descriptive studies such as comparative studies, correlation studies and case-control studies |
| IV | Evidence from expert committee reports or opinions and/or clinical experience of respected authorities |